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26 August 2025 (updated) Signs of a bad catalytic converter and how to identify them
Engine performance issues, unusual odours, and failed emissions tests are all telltale clogged catalytic converter symptoms that every driver should be aware of. By recognising these warning signs early, you can avoid costly repairs and prevent damage to other vehicle components.
How to tell if a catalytic converter is bad
A bad catalytic converter often shows several distinct faulty catalytic converter symptoms. Watch for decreased engine performance, particularly when accelerating, along with a sulphur smell similar to rotten eggs. Your vehicle may fail emissions tests and have reduced fuel efficiency, and you may also notice unusual rattling sounds coming from underneath the vehicle. The check engine light may illuminate, often with specific trouble codes related to cat efficiency.
Catalytic converter rattle
A rattling catalytic converter produces a distinctive metallic sound that gets louder during acceleration or when the engine is under load. This noise often indicates internal damage where the honeycomb catalyst material has broken apart and is bouncing around inside the converter housing. The rattle may start intermittently but usually worsens over time if ignored. While the car might still run, ignoring this symptom can lead to complete converter failure, decreased engine performance, and potential damage to other exhaust components. AUTODOC experts recommend immediate inspection by a qualified mechanic when these sounds occur.
Your engine light is on
The engine light illuminating is often a sign of a failing catalytic converter, triggering specific diagnostic trouble codes in your vehicle's on-board computer. The light usually comes on when the converter is no longer processing harmful exhaust emissions efficiently, which the oxygen sensors detect and report. While other issues can trigger the engine light, codes P0420, P0421, or P0430 specifically point to cat efficiency problems. Professional diagnostic scanning can confirm whether your catalytic converter requires cleaning, repair, or needs to be installed with a new catalytic converter before more costly complications arise.
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Your car accelerates more slowly than usual
If your car is accelerating more slowly than usual, it may have a clogged or failing cat, which can restrict exhaust flow and create dangerous back pressure in your engine system. This decline in performance often begins subtly but worsens progressively, with the vehicle feeling particularly sluggish during acceleration or when climbing hills. As exhaust gases cannot flow freely through the impaired converter, your engine struggles to “breathe” properly, which reduces power output and responsiveness. You may also notice increased fuel consumption, as your engine works harder to overcome this restriction.
Strange exhaust smell could mean a bad catalytic converter
A distinctive warning sign of catalytic converter failure is a peculiar exhaust odour, often compared to the smell of rotten eggs or sulphur. This occurs when the converter can no longer process hydrogen sulfide properly, allowing this smelly compound to escape unprocessed. The odour is usually noticeable around the exterior of the vehicle and may sometimes seep into the cabin. As the converter deteriorates, it fails to complete the chemical reactions needed to neutralise harmful emissions, resulting in these unpleasant smells.
Your engine misfires during driving
The engine misfiring while driving can indicate a failing catalytic converter that is restricting the flow of exhaust gases. When exhaust gases cannot escape efficiently, they create back pressure, disrupting the engine's combustion cycle. These misfires usually show themselves as hesitation, bucking, or jerking when accelerating. A severely clogged converter can force unburned fuel back into the combustion chamber, causing irregular firing patterns and potentially damaging engine components.
Common causes of a clogged catalytic converter
Over time, a catalytic converter may become blocked due to several factors related to engine or exhaust system performance. Typical reasons for this include:
- Frequent short-distance driving prevents the converter from reaching its optimal operating temperature
- Engine performance issues, such as misfiring spark plugs, allowing unburned fuel into the exhaust system
- Oil or coolant leaks entering the exhaust stream and coating the catalyst material
- Using leaded gasoline or fuel additives that contaminate the converter
- Age-related deterioration of the honeycomb catalyst structure
- Oxygen sensor failures that create improper fuel mixtures
Carbon deposit build-up in the catalytic converter
Carbon deposits build up in catalytic converters when unburned fuel enters the exhaust system and forms a sooty residue on the honeycomb catalyst material. This residue gradually restricts exhaust flow, reducing the converter's efficiency and eventually blocking passages completely. Common causes include rich fuel mixtures, frequent short journeys, and engine misfires. Once significant carbon build-up has occurred, the converter will require professional cleaning or replacement to restore proper exhaust function.
Visible damage
Visible damage to catalytic converters may take the form of dents, crushed sections or corrosion on the outer casing. These defects are often caused by impacts with road debris, speed bumps, or exposure to extreme heat. Tampering or cutting of the converter's mounting brackets may indicate attempted theft. Rust along seams or connection points suggests deterioration that could compromise the unit's structural integrity and exhaust-sealing capability.
Catalytic converter problems caused by misfires and excess fuel
Engine misfires and excess fuel delivery can damage catalytic converters by flooding them with unburned hydrocarbons that ignite inside its ceramic honeycomb structure, creating extremely high temperatures that melt internal components. This thermal damage is often irreversible, resulting in deterioration, clogging, and eventual failure of the catalyst. Additionally, the excess carbon deposits from unburned fuel gradually coat the active surfaces, which severely reduces the converter's ability to process harmful emissions.
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How to check a catalytic converter
If you think your catalytic converter might be faulty, there are a few methods you can use to assess its condition:
- Perform a visual inspection to check for any external damage, rust or discolouration that indicates overheating
- With the engine running, listen for rattling sounds that suggest internal honeycomb breakdown
- Use an infrared thermometer to measure temperature differences; a properly functioning catalytic converter should be hotter at the outlet than the inlet
- Monitor engine performance during acceleration for signs of power loss or hesitation.
- Use an emissions analyser to check exhaust gas readings for unusual levels of hydrocarbons or carbon monoxide
- Tap the converter gently with a rubber mallet while cold; hollow or rattling sounds suggest internal damage
- Use a vacuum gauge to test for excessive back pressure in the exhaust system
- Use an OBD scanner to scan for diagnostic trouble codes
Why do you need to replace a broken catalytic converter?
A damaged cat fails to properly convert harmful emissions like carbon monoxide and nitrogen oxides into less harmful gases, and so significantly increases your vehicle's pollution output. This malfunction usually activates the check engine light and can result in your vehicle failing emissions tests, making it illegal to drive in many regions. On top of this, a broken catalytic converter restricts exhaust flow, which reduces engine efficiency and increases fuel consumption. It can also cause excessive exhaust heat build-up, which could affect other engine components.
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FAQs
What are the symptoms of a leaking catalytic converter?
Symptoms of a leaking catalytic converter include hissing or ticking sounds from the exhaust, unusual odours, visible exhaust leaks, decreased fuel efficiency, reduced engine performance, and an illuminated check engine light.
What are the symptoms of a cracked catalytic converter I should look out for?
A cracked cat typically produces loud exhaust noise, decreased engine performance, a strong sulphuric smell, and an illuminated check engine light. You may also notice increased emissions, rattling, and visible damage upon inspection.
What are the symptoms of a hole in the catalytic converter?
A hole in your catalytic converter causes excessive exhaust noise, reduced engine power, poor fuel economy, and triggers the check engine light. You might notice a strong exhaust smell and see exhaust leaks on inspection.
